T x;
T foo(T, T);
bind1st(ptr_fun (foo), x)
creates a function object that takes an argument of type T const&. This
does not work if T is already a reference type like int const&. So my
first problem is with the &. My second problem is with the const. Why
should bind1st change the constness of the second argument of the
function*?
I am currently using boost::bind instead, but I would rather not depend
on boost for all my programs...
6  2215 
Marc wrote:
T x;
T foo(T, T);
bind1st(ptr_fun (foo), x)
creates a function object that takes an argument of type T const&. This
does not work if T is already a reference type like int const&. So my
first problem is with the &. My second problem is with the const. Why
should bind1st change the constness of the second argument of the
function ?
If instead of bind1st(Op const& op, T const& y) we had
bind1st(Op const& op, T& y), it would cause a problem deducing a
const reference template argument when used with, e.g. a temporary
or a numeric literal:
bind1st(ptr_fun (foo), 13);
The deduced type of template parameter T would be int, not const int,
T& would not bind to the argument, and the above would fail to compile.
I am currently using boost::bind instead, but I would rather not depend
on boost for all my programs...
You could change the parameters of foo from references to pointers.
When I was experimenting with this problem (and couldn't use boost)
I came up with a rather uncouth alternative of duplicating the standard
bind1st for the case when the template parameters are references (I've
never used it for real though):
template <class Op>
class refbinder1st : public unary_function< typename Op::second_argu ment_type,
typename Op::result_type > {
protected:
Op op_;
typename Op::first_argum ent_type val_;
public:
refbinder1st(co nst Op& op,
const typename Op::first_argum ent_type val) : op_(op),
val_(val) {}
typename Op::result_type operator ()(
const typename Op::second_argu ment_type arg) const {
return op_(val_, arg);
}
};
// Both arguments of Op should be references or very small objects
template <class Op, typename T>
refbinder1st<Op > refbind1st(cons t Op& op, T v_1st) {
return refbinder1st<Op >(op, typename Op::first_argum ent_type(v_1st) );
}
Denis
On Mon, 5 Jul 2004 18:21:50 +0000 (UTC), Marc <Ma***********@ Loria.Fr>
wrote: T x;
T foo(T, T);
bind1st(ptr_fun (foo), x)
creates a function object that takes an argument of type T const&. This
does not work if T is already a reference type like int const&. So my
first problem is with the &. My second problem is with the const. Why
should bind1st change the constness of the second argument of the
function*?
Just the way it is written. Perfect argument passing is an unsolved
problem of C++, although the next version of the standard is going to
address it I think. In the absense of perfect argument passing, old
code used const references. Some new code uses more sophisticated
schemes, such as the call_traits in boost, or even rvalue/lvalue/const
resolving stuff (e.g. Mojo).
I am currently using boost::bind instead, but I would rather not depend
on boost for all my programs...
Boost contains too much essential stuff not to depend on it. I'd just
end up rewriting half of it if I couldn't use it. However, you can
just lift out the <boost/functional.hpp> header for a replacement
<functional> header that works much better.
Tom
--
C++ FAQ: http://www.parashift.com/c++-faq-lite/
C FAQ: http://www.eskimo.com/~scs/C-faq/top.html
Denis Remezov wrote: If instead of bind1st(Op const& op, T const& y) we had
bind1st(Op const& op, T& y), it would cause a problem deducing a
const reference template argument when used with, e.g. a temporary
or a numeric literal:
bind1st(ptr_fun (foo), 13);
The deduced type of template parameter T would be int, not const int,
T& would not bind to the argument, and the above would fail to compile.
I thought that it could be overloaded with a const and a non-const
versions. But I don't have time to check.
When I was experimenting with this problem (and couldn't use boost)
I came up with a rather uncouth alternative of duplicating the standard
bind1st for the case when the template parameters are references (I've
never used it for real though):
That is interesting, and it seems to work fine. Thank you for your
answer.
tom_usenet wrote: Just the way it is written. Perfect argument passing is an unsolved
problem of C++, although the next version of the standard is going to
address it I think.
Do you have a reference on this "perfect argument passing" issue*?
Boost contains too much essential stuff not to depend on it. I'd just
end up rewriting half of it if I couldn't use it. However, you can
just lift out the <boost/functional.hpp> header for a replacement
<functional> header that works much better.
Cool, I had not seen this in boost. I hope the next standard benefits
from it.
Thank you for your answer.
On Tue, 6 Jul 2004 13:00:43 +0000 (UTC), Marc <Ma***********@ Loria.Fr>
wrote: tom_usenet wrote:
Just the way it is written. Perfect argument passing is an unsolved
problem of C++, although the next version of the standard is going to
address it I think.
Do you have a reference on this "perfect argument passing" issue*? http://www.open-std.org/jtc1/sc22/wg...2002/n1385.htm Boost contains too much essential stuff not to depend on it. I'd just
end up rewriting half of it if I couldn't use it. However, you can
just lift out the <boost/functional.hpp> header for a replacement
<functional> header that works much better.
Cool, I had not seen this in boost. I hope the next standard benefits
from it.
Unlikely I think, but boost::bind is part of the standard library
technical report (so it will be std::tr1::bind, and should come with
your compiler): http://www.open-std.org/jtc1/sc22/wg...al_report.html
It's due in the next year.
Tom
--
C++ FAQ: http://www.parashift.com/c++-faq-lite/
C FAQ: http://www.eskimo.com/~scs/C-faq/top.html
Marc wrote:
Denis Remezov wrote:
If instead of bind1st(Op const& op, T const& y) we had
bind1st(Op const& op, T& y), it would cause a problem deducing a
const reference template argument when used with, e.g. a temporary
or a numeric literal:
bind1st(ptr_fun (foo), 13);
The deduced type of template parameter T would be int, not const int,
T& would not bind to the argument, and the above would fail to compile.
I thought that it could be overloaded with a const and a non-const
versions. But I don't have time to check.
Yes, but the problem is the automatic template parameter deduction:
template <typename T>
void f(T& v) {
}
int main() {
const int a=13;
f(a); //all right
f<int const>(13); //all right as well
f(13); //error
}
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